To drive a solenoid, there has been used a solenoid drive circuit that passes a current with the use of a semiconductor element such as an FET (Field Effect Transistor). For example, a linear solenoid that applies a voltage to an exciting coil and generates a magnetic force to directly provide a movable iron core with linear motion is capable of linearly controlling hydraulic pressure in response to a control current provided by the solenoid drive circuit and is frequently used in, for example, a car electronics field.
For example, Japanese Unexamined Patent Application Publication No. 2005-150550 describes a solenoid drive apparatus that precisely controls a solenoid with good response. This solenoid drive apparatus includes a DC power source, a switching element and solenoid connected to the DC power source in series, a PWM signal generation circuit to provide a control terminal of the switching element with a control signal and control ON/OFF of the switching element, and a control circuit configured to output a solenoid current command value to the PWM signal generation circuit thereby control the PWM signal generation circuit. The control circuit includes a current detector to detect a current passing through the solenoid and output a voltage value corresponding to the detected current, an amplifier to amplify the voltage value detected by the current detector, an integrator to integrate the amplified output from the amplifier, an operational comparison circuit to compare the integrated value from the integrator held in a hold circuit with a solenoid current target value and output the comparison result as the solenoid current command value to the PWM signal generation circuit, and a reset circuit to reset the output of the integrator.
The PWM signal generation circuit provides the control terminal of the switching element with a control signal having a pulse width corresponding to the solenoid current command value from the operational comparison circuit and provides the reset circuit with a reset signal whenever providing the switching element with the control signal, to reset the output of the integrator in synchronization with the control signal. Since the reset circuit resets the output of the integrator to zero in synchronization with the control signal from the PWM signal generation circuit, the operational comparison circuit compares, at every cycle of the control signal from the PWM signal generation circuit, an integrated value of the integrator with a target current value and outputs a solenoid current command value to the PWM signal generation circuit.
Accordingly, the related art can precisely control the solenoid in response to a current passed to the solenoid and detected by the current detector. Even if the current detector detects a voltage value containing noise caused by, for example, the switching operation of a MOSFET, or a fluctuating voltage value caused by a sudden change in a power source voltage and even if the amplifier amplifies such a noise-containing voltage value, the integrator smoothes the noise in the output of the amplifier so that the noise may not cause a serious error. Due to this, there is no need of improving the accuracy of solenoid control by frequently conducting sampling at short intervals with the hold circuit. Namely, there will be no increase in the number of operations to increase operation load on the operational comparison circuit.
The solenoid drive apparatus of the above-mentioned document compares a solenoid current target value calculated from output signals of various sensors with a solenoid current detected value, and based on a result of the comparison, controls the PWM signal generation circuit. It will be possible to use, for example, an external microcomputer to set the target value. In this case, the external microcomputer is capable of not only setting the target value but also transmitting information about the functions and setting of the solenoid drive apparatus, and therefore, is convenient. In addition, the external microcomputer can receive state information, such as overvoltage information and overheat information, from the solenoid drive apparatus. Accordingly, the external microcomputer or an operator of the microcomputer may take a measure appropriate for a situation to quickly and flexibly deal with the situation.
To achieve such communication with the external microcomputer, serial communication is generally used. The serial communication uses a single signal line to transmit/receive a bit of data per a clock and has advantages that it employs a small number of signal lines and can conduct long-distance communication.
FIG. 1 is a block diagram illustrating a conventional transmission circuit 11 used for standard serial communication. A transmit buffer 25 receives IC internal data to be transmitted and outputs the data as parallel data to a shift register 23. The shift register 23 converts the parallel data from the transmit buffer 25 into a serial signal and outputs the signal at predetermined timing. At this time, to surely output the IC internal data to the shift register 23, the transmit buffer 25 latches the parallel data, and then, outputs the same. With this, data inputted to the transmit buffer 25 during a period from the preceding serial communication conducted by the shift register 23 to the next serial signal output by the shift register 23 can be transmitted from the transmit buffer 25 to the shift register 23 without omission.